1. Field of the Invention
The present invention relates to a process for producing an .alpha., .beta.-unsaturated carboxylic acid, and more particularly, to a process for producing an .alpha., .beta.-unsaturated carboxylic acid in the presence of an improved solid acid catalyst.
.alpha., .beta.-Unsaturated carboxylic acids are used as starting materials for preparing .alpha., .beta.-unsaturated carboxylic acid esters. Among them, methacrylic acid is a starting material for producing methyl methacrylate from which poly(methyl methacrylate) exhibiting high weatherability and transparency can be prepared, and therefore, is very useful in industry.
2. Description of the Related Art
Heretofore, there are known processes for producing .alpha., .beta.-unsaturated carboxylic acids, for example, Japanese Patent Publication No. Sho 63-10940 (U.S. Pat. No. 4,464,539) discloses a method for producing an .alpha., .beta.-unsaturated carboxylic acid which comprises bringing water and an .alpha.-hydroxycarboxylic acid amide prepared by hydration of cyanohydrin into contact with a solid acid catalyst. In this case, as a representative solid acid catalyst, there may be used a catalyst containing a phosphoric acid salt such as lanthanum phosphate, cerium phosphate and the like.
As a result, for example, by using a rare earth metal phosphate of a hexagonal system structure produced by a conventional method as a solid acid catalyst, there can be obtained methacrylic acid in a 75-85 mole % yield from a 4.8 mole % aqeous solution of .alpha.-hydroxyisobutyramide at a liquid hourly space velocity (LHSV) of 0.2-1.3 hr.sup.-1 and at a reaction temperature of 300.degree.-400.degree. C.
However, in an industrial production, the .alpha., .beta.-unsaturated carboxylic acid obtained by the above-mentioned method is continuously used as a starting material for an .alpha., .beta.-unsaturated carboxylic acid ester, and therefore, there has been demanded a solid acid catalyst capable of keeping a high catalytic activity at a lower reaction temperature for a long period of time under a higher catalytic load.